config: add 'git_config_string' to refactor string config variables.

config: add 'git_config_string' to refactor string config variables.

In many places we just check if a value from the config file is not
NULL, then we duplicate it and return 0. This patch introduces the new
'git_config_string' function to do that.

This function is also used to refactor some code in 'config.c'.
Refactoring other files is left for other patches.

Also not all the code in "config.c" is refactored, because the function
takes a "const char **" as its first parameter, but in many places a
"char *" is used instead of a "const char *". (And C does not allow
using a "char **" instead of a "const char **" without a warning.)

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>